Non-oriented electrical steels are widely used as the magnetic core material in a variety of electrical machinery and devices, particularly in motors where low core loss and high magnetic permeability in all directions of the strip are desired. The present invention relates to a method for producing a non-oriented electrical steel with low core loss and high magnetic permeability whereby the steel is produced from a steel melt which is cast as a thin strip, cooled, hot rolled and/or cold rolled into a finished strip. The finished strip is further subjected to at least one annealing treatment wherein the magnetic properties are developed, making the steel strip of the present invention suitable for use in electrical machinery such as motors or transformers.
The magnetic properties of non-oriented electrical steels can be affected by finished strip thickness, volume resistivity, grain size, purity and crystallographic texture of the finished strip. The core loss caused by eddy currents can be made lower by reducing the thickness of the finished steel strip, increasing the alloy content of the steel strip to increase the volume resistivity or both in combination.
Established methods for producing non-oriented electrical steels with conventional processing (thick slab casting, slab reheating, hot rolling and hot band annealing) use typical but non-limiting alloy additions of silicon, aluminum, manganese and phosphorus with, preferably, compositions which provide for a fully ferritic microstructure within which any residual nitrogen is in the form of large inclusions. Non-oriented electrical steels may contain up to about 6.5% silicon, up to about 3% aluminum, up to about 0.05% carbon (which must be reduced to below about 0.003% during processing to prevent magnetic aging), up to about 0.01% nitrogen, up to about 0.01% sulfur and balance iron with a small amount of impurities incidental to the method of steel making. Non-oriented electrical steels, including those generally referred to as motor lamination steels, are differentiated by proportions of additions such as silicon, aluminum and like elements made to increase the volume resistivity of the steel. Steels containing less than about 0.5% silicon and other additions to provide a volume resistivity of about 20 μΩ-cm can be generally classified as motor lamination steels; steels containing about 0.5 to about 1.5% silicon or other additions to provide a volume resistivity of from about 20 μΩ-cm to about 30 μΩ-cm can be generally classified low-silicon steels; steels containing about 1.5 to about 3.0% silicon or other additions to provide a volume resistivity of from about 30 μΩ-cm to about 45 μΩ-cm can be generally classified as intermediate-silicon steels; and, lastly, steels containing more than about 3.5% silicon or other additions to provide a volume resistivity greater than about 45 μΩ-cm can be generally classified as high-silicon steels. Typically, these steels contain aluminum additions as well. Silicon and aluminum greatly increase the stability of the ferrite phase, thereby steels containing in excess of about 2.5% (silicon+aluminum) are ferritic, that is, no austenite/ferrite phase transformation will occur during heating or cooling. Such alloying additions increase volume resistivity and suppress eddy currents during AC magnetization, thereby lowering core loss. These additions also improve the punching characteristics of the steel by increasing the hardness. Conversely, increasing the alloy content makes the steel more difficult to manufacture owing to the added cost of alloying and increased brittleness, particularly when large amounts of silicon are employed.
Achieving a suitably large grain size in the finish rolled and annealed strip is desired to provide minimal hysteresis loss. The purity of the finish rolled and annealed strip can have a significant effect on core loss since the presence of a dispersed phase, inclusions and/or precipitates can inhibit grain growth during annealing, preventing the formation of an appropriately large grain size and orientation and, thereby, producing higher core loss and lower magnetic permeability in the final product form. Also, inclusions and/or precipitates in the finish annealed steel hinder domain wall motion during AC magnetization, further degrading the magnetic properties. As noted above, the crystallographic texture of the finished strip, that is, the distribution of the orientations of the crystal grains comprising the electrical steel strip, is very important in determining the core loss and magnetic permeability. The <100> and <110> texture components as defined by Millers indices have the highest magnetic permeability; conversely, the <111> type texture component has the lowest magnetic permeability.
Non-oriented electrical steels are generally provided in two forms, commonly referred to as “semi-processed” or “fully-processed” steels. “Semi-processed” infers the product must be annealed before use to develop the proper grain size and texture, relieve fabrication stresses and, if needed, provide appropriately low carbon levels to avoid aging. “Fully-processed” infers that the magnetic properties have been fully developed prior to the fabrication of the strip into laminations, that is, the grain size and texture have been established and the carbon content has been reduced to about 0.003% or less to prevent magnetic aging. These grades do not require annealing after fabrication into laminations unless so desired to relieve fabrication stresses. Non-oriented electrical steels are predominantly used in rotating devices, such as motors or generators, where uniform magnetic properties are desired in all directions with respect to the strip rolling direction, or where the cost of a grain oriented electrical steel is not justified.
Non-oriented electrical steels differ from grain oriented electrical steels since grain oriented electrical steels are processed so as to develop a preferred orientation by a process known as secondary grain growth (or secondary recrystallization). Secondary grain growth results in the electrical steel having extremely directional magnetic properties with respect to the strip rolling direction, making grain oriented electrical steels suitable for applications where directional properties are desired, such as in transformers.
Commercially available non-oriented electrical steels are typically broken into two classifications: cold rolled motor lamination steels (“CRML”) and cold rolled non-oriented electrical steels (“CRNO”). CRML is generally used in applications where the requirement for very low core losses is difficult to justify economically. Such applications typically require that the non-oriented electrical steel have a maximum core loss of about 4 W/# (watts/pound) (about 8.8 watts/kg) and a minimum magnetic permeability of about 1500 G/Oe (Gauss/Oersted) measured at 1.5 T and 60 Hz. In such applications, the steel strip used is typically processed to a nominal thickness of about 0.018 inch (about 0.45 mm) to about 0.030 inch (about 0.76 mm). CRNO is generally used in more demanding applications where better magnetic properties are required. Such applications typically require that the non-oriented electrical steel has a maximum core loss of about 2 W/# (about 4.4 W/kg) and a minimum magnetic permeability of about 2000 G/Oe measured at 1.5 T and 60 Hz. In such applications, the steel strip is typically processed to a nominal thickness of about 0.008 inch (about 0.20 mm) to about 0.025 inch (about 0.63 mm).
None of the previous methods teach or suggest the method of the present invention in which the non-oriented electrical steel is made from a cast strip to meet the above mentioned magnetic property requirements in an economical manner.